The latest generation of speed cameras which record number plates and take pictures of drivers could be illegal, a Government watchdog has warned.

Sir Andrew Leggatt, the Surveillance Commissioner, has said that new laws are needed to prevent the evidence the cameras provide from being successfully challenged in the courts.

According to Sir Andrew, the cameras could be classed as "covert surveillance" and could be declared inadmissable. "It probably also amounts to the obtaining of private information about any such person, whether or not that person has been identified for the purposes of the investigation," he said.

"The procedure will therefore be vulnerable to challenge unless it is authorised."

Sir Andrew warned there could be "human rights" issues if the material was used to build up a picture of the movement of particular people or vehicles.

His reservations could have serious implications for the newest cameras, which aim to stop motorists ducking responsibility for speeding by claiming that somebody else was at the wheel.

"Speed cameras used to be rear-facing but now they look at the front of the car to try to avoid people trying to pervert the course of justice by claiming they weren't driving," said Paul Watters, of the AA Motoring Trust. "I am a supporter but they do have to be legal."

According to Paul Smith, a campaigner against speed cameras, the use of front-facing cameras capable of identifying not only the number plate but also the driver is on the increase.

"They have put 10 in Essex," he said. "This is a growing trend, it is their way of saying there is no escape."

Sir Andrew said that signs telling motorists that cameras are in place would not necessarily prevent a legal challenge because the driver would not have been warned that he or she could also have his picture taken.

Such cameras underpin a raft of Government policies, from tracking the movements of criminals to identifying uninsured vehicles and enforcing speed limits. They are also used by councils and private companies to issue parking tickets.

Transport for London uses them for enforcing the congestion charge and they could form part of the technology employed in any urban or motorway road pricing system.

Even this information could be subject to challenge in the courts unless the law is updated to keep up with the rapid advance of technology, Sir Andrew said.

A Home Office spokesman said last night: "We have noted the comments in the Chief Surveillance Commissioner's annual report. There is a review currently under way."